The present invention relates to porous ceramic articles, and more particularly to porous ceramic articles for use as filters for removing particulates from diesel exhaust gases.
"Diesel exhaust traps" are filtering devices designed to remove particulate material (e.g., soot) from the exhaust of automobile or truck diesel engines, a need dictated to a large extent by increasingly stringent governmental regulations in the United States and European countries regarding maximum allowable particulates in automotive emission gases. Generally, soot trapped by the filtering devices is then periodically combusted in the filter so as to regenerate the filtering surfaces, the combustion being initiated, for example, by electrical means or fuel burner devices associated with the overall trap design, or by variable operation of the engine itself or other means to provide to the filter an exhaust stream sufficiently hot to initiate the combustion process.
As a consequence of the generalized designs for particulate traps of this type, the filtering element is required to have a number of properties. Obviously, it is essential that the filtering element exhibit porosity which permits trapping of particulates, but at the same time it is essential that the construction of the filter be such that exhaust gases can travel relatively easily therethrough without creation of any significant degree of back pressure. Moreover, it is necessary that the filtering element present a substantial filtering surface per unit length, area or volume so as to permit the element to be fabricated in an overall size consistent with the constraints imposed by the size of the exhaust systems of diesel-powered automobiles and trucks. Because of its exposure to hot exhaust gases, and even higher temperatures during a combustion/regeneration cycle, the filtering element also necessarily must possess structural and dimensional stability under such conditions.
The prior art has sought to provide filtering elements for diesel particulate traps possessing these physical characteristics by resort to a variety of materials. Early efforts relied upon stainless steel mesh or coils of fibrous metallic wire as filtering materials, as evidenced by U.S. Pat. Nos. 3,937,015 and 4,270,936, respectively. More recent efforts have concentrated upon ceramic materials since they generally possess excellent structural and dimensional stability under stringent (i.e., high temperature) operating conditions, with the requirement that the trap exhibit porosity effective to filter soot from exhaust gases being accomplished by various compositional and processing techniques. Most notable in these efforts has been the utilization of so-called ceramic monolithic honeycomb filtering elements as described, for example, in U.S. Pat. Nos. 4,276,071 and 4,364,761 assigned to General Motors Corp.; U.S. Pat. Nos. 4,329,162; 4,415,344; 4,416,675; 4,416,676; 4,417,908; 4,419,108 and 4,420,316 assigned to Corning Glass; and U.S. Pat. Nos. 4,283,210; 4,293,357; 4,307,198; 4,340,403; and 4,364,760 assigned to NGK Insulators. Essentially, these elements consist of a monolithic ceramic having a multitude of internal parallel chambers separated by thin porous ceramic internal walls, with a number of the chambers being end-sealed so as to force particulate-containing exhaust gas to traverse across a porous wall before exiting the element. Generally, these elements are formed by an extrusion process, and materials are included in the compositions which are burned out during the firing process so as to provide the requisite porosity in the internal filtering surfaces. In another process, as reflected in U.S. Pat. No. 4,363,644 assigned to Nippon Soken, foamed, structural polyurethane systems are utilized in admixture with inorganic materials in processes wherein the polyurethane burns out during firing so as to leave behind a ceramic structure having a variety of interconnected open cells for trapping particulates.
While the structural and dimensional properties of ceramics generally lend themselves well to utilization as the material from which filter elements for diesel traps are constructed, it is not an easy or inexpensive matter to achieve from ceramic materials elements possessing the porosity needed to effectively and efficiently filter soot as well as permit exhaust gas flow without substantial build-up of back pressure. For example, in the highly permeable reticulated foam filters in the art, a condition can occur ("blow-off") in which soot already collected in the filter can be dislodged as a consequence either of excessive build-up or sudden increase in the velocity of the exhaust gas stream. Of additional importance, efforts toward optimizing the geometry of diesel filter trap designs (so as to facilitate inclusion of the filter in the exhaust area of a vehicle, or to maximize filtration, or to facilitate regeneration or removal of the filter element) can be severely limited by the inability to produce such shapes efficiently (or at all) utilizing ceramics.
In response to the foregoing limitations in current technology, the present invention provides compositions and methods for making porous refractory ceramic filters for use in removing particulates from diesel exhaust gases. As will be seen, the invention employs moldable/extrudable compositions which enable any variety of shapes to be provided, methods for rendering those shapes more refractory and resistant to thermal shock upon firing, and methods for providing thin porous ceramic membrane layers on the fired article to enhance filtering ability.